1. Field of the Invention
This invention relates to magnetomechanical electronic article surveillance systems and methods, and more particularly to the generation and detection of sideband signals from a magnetomechanical marker.
2. Description of the Related Art
Electronic article surveillance (EAS) systems are well known for the prevention or deterrence of unauthorized removal of articles from a controlled area. In a typical EAS system, markers designed to interact with an electromagnetic field located at the exits of the controlled area are attached to articles to be protected. If a marker is brought into the electromagnetic field or "interrogation zone", the presence of the marker is detected and appropriate action is taken, such as generating an alarm.
Several types of EAS systems and markers are presently known. In one type, the marker includes either an antenna and diode, or an antenna and capacitors forming a resonant circuit. When placed in an electromagnetic field transmitted by the interrogation apparatus, the marker having an antenna and diode generates harmonics of the interrogation frequency in the receive antenna; the resonant circuit marker causes an increase in absorption of the transmitted signal so as to reduce the signal in the receiving coil. Detection of the harmonics or the signal level change in the receive coil indicates the presence of the marker.
One of the problems with harmonic generating markers and resonant circuit markers is the difficulty with detection at remote distances. Another problem with harmonic generating and resonant circuit markers is the difficulty in distinguishing the marker signal from pseudo signals generated by other items such as belt buckles, pens, hair clips, and other metallic objects.
U.S. Pat. No. 4,660,025 discloses an improved harmonic generating marker utilizing a magnetic material having a magnetic hysteresis loop that exhibits a large Barkhausen discontinuity. The magnetic material, when exposed to an external magnetic field whose field strength in the direction opposing the instantaneous magnetic polarization of the material exceeds a predetermined threshold value, results in a regenerative reversal of the magnetic polarization of the material. The result of utilizing markers having magnetic material exhibiting a large Barkhausen discontinuity is the production of high order harmonics having amplitudes that are more readily detected. However, false alarms are still possible utilizing these improved harmonic generating markers.
Harmonic generating markers rely on non-linear behavior of the magnetic materials to generate the harmonic signals needed for detection. A more robust EAS system utilizes magnetomechanical or magnetoacoustic markers in which magnetic resonators operate in a linear magnetic response region.
U.S. Pat. Nos. 4,510,489 and 4,510,490 each disclose an electronic article surveillance (EAS) system and associated magnetomechanical marker. The magnetomechanical marker includes a resonator element made of a magnetostrictive material, which in the presence of a biasing magnetic field, resonates in response to a specific frequency. The biasing magnetic field is typically provided by a ferromagnetic element disposed adjacent the magnetostrictive material. Upon being magnetized, the ferromagnetic element provides a biasing magnetic field that enables the magnetostrictive material to resonate at its preselected resonance frequency. The marker is detected by detecting the change in coupling between an interrogating coil and a receiving coil at the marker's resonant frequency.
Because the marker is interrogated and detected at the marker's resonant frequency, the transmitted interrogation frequency interferes with detection of the marker. Therefore, a burst or pulsed magnetomechanical EAS system is preferred. In the pulsed system, a transmitter generates a signal at a preselected frequency, such as 58 kHz, for a fixed duration to excite the marker. The receiver is disabled for the transmit period. The receiver is then activated to detect the resonant envelope of the marker as it decays over time, commonly referred to as "ring-down". A marker having a high quality factor (Q) response is required for good detection in a pulsed system, resulting in few false alarms and detection from remote distances. While, a pulsed magnetomechanical is the highest quality and highest functioning EAS system available to date, there is room for improvement.
After a transmit pulse is generated, the receiver typically includes an initialization period after activation which causes the receiver's detection window to be delayed slightly. In addition, due to the finite length of the transmit pulse, the marker may not have sufficient time to build up full energy before the transmitter is deactivated, and the marker may begin to ring-down from a lower energy level. The detection window is thus shifted to a time when the marker has already lost some of its available stored energy, making detection more difficult. An improved signal generation and detection method for magnetomechanical markers is desired.